A kinetics model was described for the surface electron transfer at illuminated colloidal semiconductor particles to discuss the effect of the acceptor concentration. The principle is a modification of the recombination model which was previously proposed by the authors to understand the dependence of the electron-transfer quantum yield on the photon fluence. The modified recombination model was successfully applied to the experimental results, where the quantum yields for the electron transfer from colloidal CdS to methylviologen were obtained at various fluences as a function of acceptor concentration. The new model reveals that the Langmuir-type relationship between the reaction yield and the reactant concentration is not applicable at higher photon fluences.